Amyloidosis refers to a pathological condition characterized by the presence of amyloid fibrils. Amyloid is a generic term referring to a group of diverse but specific protein deposits (intracellular or extracellular) which are seen in a number of different diseases. Though diverse in their occurrence, all amyloid deposits have common morphologic properties, stain with specific dyes (e.g., Congo red), and have a characteristic red-green birefringent appearance in polarized light after staining. They also share common ultrastructural features and common X-ray diffraction and infrared spectra.
Amyloid-related diseases can either be restricted to one organ or spread to several organs. The first instance is referred to as “localized amyloidosis” while the second is referred to as “systemic amyloidosis.”
Some amyloid diseases can be idiopathic, but most of these diseases appear as a complication of a previously existing disorder. For example, primary amyloidosis (AL amyloid) can appear without any other pathology or can follow plasma cell dyscrasia or multiple myeloma.
Secondary amyloidosis is usually seen associated with chronic infection (such as tuberculosis) or chronic inflammation (such as rheumatoid arthritis). A familial form of secondary amyloidosis is also seen in other types of familial amyloidosis, e.g., Familial Mediterranean Fever (FMF). This familial type of amyloidosis is genetically inherited and is found in specific population groups. In both primary and secondary amyloidosis, deposits are found in several organs and are thus considered systemic amyloid diseases.
“Localized amyloidoses” are those that tend to involve a single organ system. Different amyloids are also characterized by the type of protein present in the deposit. For example, neurodegenerative diseases such as scrapie, bovine spongiform encephalitis, Creutzfeldt-Jakob disease, and the like are characterized by the appearance and accumulation of a protease-resistant form of a prion protein (referred to as AScr or PrP-27) in the central nervous system. Similarly, Alzheimer's disease, another neurodegenerative disorder, is characterized by neuritic plaques and neurofibrillary tangles. In this case, the amyloid plaques found in the parenchyma and the blood vessel is formed by the deposition of fibrillar Aβ amyloid protein. Other diseases such as adult-onset diabetes (type II diabetes) are characterized by the localized accumulation of amyloid fibrils in the pancreas.
Once these amyloids have formed, there is no known, widely accepted therapy or treatment which significantly dissolves amyloid deposits, in situ prevents further amyloid deposition or prevents the initiation of amyloid deposition.
Each amyloidogenic protein has the ability to undergo a conformational change and to organize into β-sheets and form insoluble fibrils which may be deposited extracellularly or intracellularly. Each amyloidogenic protein, although different in amino acid sequence, has the same property of forming fibrils and binding to other elements such as proteoglycan, amyloid P and complement component. Moreover, each amyloidogenic protein has amino acid sequences which, although different, show similarities such as regions with the ability to bind to the glycosaminoglycan (GAG) portion of proteoglycan (referred to as the GAG binding site) as well as other regions which promote α-sheet formation. Proteoglycans are macromolecules of various sizes and structures that are distributed almost everywhere in the body. They can be found in the intracellular compartment, on the surface of cells, and as part of the extracellular matrix. The basic structure of all proteoglycans is comprised of a core protein and at least one, but frequently more, polysaccharide chains (GAGs) attached to the core protein. Many different GAGs have been discovered including chondroitin sulfate, dermatan sulfate, keratan sulfate, heparin, and hyaluronan.
In specific cases, amyloid fibrils, once deposited, can become toxic to the surrounding cells. For example, the Aβ fibrils organized as senile plaques have been shown to be associated with dead neuronal cells, dystrophic neurites, astrocytosis, and microgliosis in patients with Alzheimer's disease. When tested in vitro, oligomeric (soluble) as well as fibrillar Aβ peptide was shown to be capable of triggering an activation process of microglia (brain macrophages), which would explain the presence of microgliosis and brain inflammation found in the brain of patients with Alzheimer's disease. Both oligomeric and fibrillar Aβ peptide can also induce neuronal cell death in vitro. See, e.g., M P Lambert, et al., Proc. Natl. Acad. Sci. USA 95, 6448-53 (1998).
In another type of amyloidosis seen in patients with type II diabetes, the amyloidogenic protein IAPP, when organized in oligomeric forms or in fibrils, has been shown to induce α-islet cell toxicity in vitro. Hence, appearance of IAPP fibrils in the pancreas of type II diabetic patients contributes to the loss of the β islet cells (Langerhans) and organ dysfunction which can lead to insulinemia.
Another type of amyloidosis is related to β2 microglobulin and is found in long-term hemodialysis patients. Patients undergoing long term hemodialysis will develop β2-microglobulin fibrils in the carpal tunnel and in the collagen rich tissues in several joints. This causes severe pains, joint stiffness and swelling.
Amyloidosis is also characteristic of Alzheimer's disease. Alzheimer's disease is a devastating disease of the brain that results in progressive memory loss leading to dementia, physical disability, and death over a relatively long period of time. With the aging populations in developed countries, the number of Alzheimer's patients is reaching epidemic proportions.
People suffering from Alzheimer's disease develop a progressive dementia in adulthood, accompanied by three main structural changes in the brain: diffuse loss of neurons in multiple parts of the brain; accumulation of intracellular protein deposits termed neurofibrillary tangles; and accumulation of extracellular protein deposits termed amyloid or senile plaques, surrounded by misshapen nerve terminals (dystrophic neurites) and activated microglia (microgliosis and astrocytosis). A main constituent of these amyloid plaques is the amyloid-β peptide (β), a 39-43 amino-acid protein that is produced through cleavage of the β-amyloid precursor protein (APP). Extensive research has been conducted on the relevance of Aβ deposits in Alzheimer's disease, see, e.g., Selkoe, Trends in Cell Biology 8, 447-453 (1998). Aβ naturally arises from the metabolic processing of the amyloid precursor protein (“APP”) in the endoplasmic reticulum (“ER”), the Golgi apparatus, or the endosomal-lysosomal pathway, and most is normally secreted as a 40 (“Aβ1-40”) or 42 (“Aβ1-42”) amino acid peptide (Selkoe, Annu. Rev. Cell Biol. 10, 373-403 (1994)). A role for Aβ as a primary cause for Alzheimer's disease is supported by the presence of extracellular Aβ deposits in senile plaques of Alzheimer's disease, the increased production of Aβ in cells harboring mutant Alzheimer's disease associated genes, e.g., amyloid precursor protein, presenilin I and presenilin II; and the toxicity of extracellular soluble (oligomeric) or fibrillar Aβ to cells in culture. See, e.g., Gervais, Eur. Biopharm. Review, 40-42 (Autumn 2001); May, DDT6, 459-62 (2001). Although symptomatic treatments exist for Alzheimer's disease, this disease cannot be prevented or cured at this time.
Alzheimer's disease is characterized by diffuse and neuritic plaques, cerebral angiopathy, and neurofibrillary tangles. Plaque and blood vessel amyloid is believed to be formed by the deposition of insoluble Aβ amyloid protein, which may be described as diffuse or fibrillary. Both soluble oligomeric Aβ and fibrillar Aβ are also believed to be neurotoxic and inflammatory.
Another type of amyloidosis is cerebral amyloid angiopathy (CAA). CAA is the specific deposition of amyloid-β fibrils in the walls of leptomingeal and cortical arteries, arterioles and veins. It is commonly associated with Alzheimer's disease, Down's syndrome and normal aging, as well as with a variety of familial conditions related to stroke or dementia (see Frangione et al., Amyloid: J. Protein Folding Disord. 8, Suppl. 1, 36-42 (2001)).
Presently available therapies for treatment of β-amyloid diseases are almost entirely symptomatic, providing only temporary or partial clinical benefit. Although some pharmaceutical agents have been described that offer partial symptomatic relief, no comprehensive pharmacological therapy is currently available for the prevention or treatment of, for example, Alzheimer's disease.